In known heated platten presses of this type, which are generally used for the manufacture of chip boards, fibre boards or the like, it proves difficult to produce boards of uniform thickness. This difficulty arises for a number of reasons.
Firstly the particulate material which is initially scattered to form the mat is not always uniformly scattered so that variations in density are present in the loosely scattered mat The loosely scattered mat is usually subjected to a prepressing operation before it is introduced into the heated platten press and the aforementioned density variations are present in the prepressed mat. Furthermore, deformation of one or both of the press plattens can also occur as a result of the loads imposed by the high press pressure and also as a result of the thermal deflection which are brought about by the high operating temperatures.
The resulting inaccuracies in thickness are troublesome because they have to be removed by subjecting the surfaces of the pressed boards to a grinding process in order to achieve the required plane parallelism. This grinding procedure gives rise to high grinding losses which are undesirable for economic reasons.
Accordingly many attempts have been made to overcome the outlined problems, which arises with heated platten presses, by overdimensioning the parts which are in danger of deforming or deflecting. It is indeed possible in this way to improve the thickness tolerances of the pressed material. However, the overdimensioning of the press plattens leads to unnecessary expense and complication, and to a general overall increases in the size of the actuation mechanism etc., which can result in the press becoming economically and technically unattractive.
Attempts have been made to reduce the deformations which occur as a result of thermal expansions by the provision of canal systems for the throughflow of heating and/or cooling media in the press plattens. These systems are, however, unnecessarily complex and, in the final analysis, are insufficient to allow the consistent manufacture of particle boards of uniform thickness.
An arrangement is also known which incorporates a rigid lower press platten and a len rigid upper press platten. In this arrangement the rigid press platten is fixed and the upper press platten is advanced towards the rigid press platten by means of hydraulic actuating cylinders so as to effect initial closure of the press. Further piston-in-cylinder arrangements are then provided at the centre of the upper press platten and are actuated so as to counteract the load dependent bowing and the centre of the deformable platten. This known construction does not however make it possible to take account of the small area thickness variations which arise as a result of non-uniform scattering of the mat and only allow an improvement of the inaccuracy introduced into the pressed board as a result of bowing deflection of the upper press platten.